Using same build as you and Windows 8. Turning off Crystal Mode for Coherence seems to help but not consistently. If I exit Outlook and relaunch whatever is in Outbox sends promptly.
MS Outlook offers an inbuilt application named as Inbox Repair Tool, you should try to use it to solve your PST related issues. You can also use a third party Outlook PST repair software to fix PST corruption issues. I would like to suggest such an effective software, which can be downloaded from here: http://www.pstoutlookrepair.com/solutions/how-to-avoid-common-pst-issues-in-ms-outlook.html